I also calculate from full tank to full tank readings and they have been tallying with average meter of the car. Initially the average meter shows high readings if you top up, reset the average and drive away it used to show 15+ but for few kms and after about 50 kms the average used to settle at between 10-11 kms/hr. But now I have been driving for three days and it is staying in 13 zone. I will reconfirm on next fill up.
You do not get true average after driving 25 kms after fill up and then fill up again. It's usually full tank, note the reading and and when tank is empty again fill up and then calculate, will give you appx figure because it is impossible to empty the tank and there is still 10 lit petrol in the tank when reserve lights come up.